Method of growing a thin film onto a substrate

ABSTRACT

The present invention relates to the production of thin films. In particular, the invention concerns a method of growing a thin film onto a substrate, in which method the substrate is placed in a reaction chamber and is subjected to surface reactions of a plurality of vapor-phase reactants according to the ALD method. The present invention is based on replacing the mechanical valves conventionally used for regulating the pulsing of the reactants, which valves tend to wear and intrude metallic particles into the process flow, with an improved precursor dosing system. The invention is characterized by choking the reactant flow between the vapour-phase pulses while still allowing a minimum flow of said reactant, and redirecting the reactant at these times to an other destination than the reaction chamber. The redirection is performed with an inactive gas, which is also used for ventilating the reaction chamber between the vapour-phase pulses.

This is the U.S. national phase under 35 U.S.C. § 371 of InternationalApplication PCT/FI01/00680, filed Jul. 20, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of thin films. Inparticular, the invention concerns a method of growing a thin film ontoa substrate, in which method the substrate is placed in a reactionchamber and it is subjected to surface reactions of a plurality ofvapor-phase reactants according to the ALD method to form a thin film.

2. Description of Related Art

Conventionally, thin films are grown out using vacuum evaporationdeposition, Molecular Beam Epitaxy (MBE) and other similar vacuumdeposition techniques, different variants of Chemical Vapor Deposition(CVD) (including low-pressure and metallo-organic CVD andplasma-enhanced CVD) or, alternatively, the above-mentioned depositionprocess based on alternate surface reactions, known in the art as theAtomic Layer Deposition, in the following abbreviated ALD, formerly alsocalled Atomic Layer Epitaxy or “ALE”. Commercially available equipmentand processes are supplied by ASM Microchemistry, Espoo, Finland, underthe trade mark ALCVD™.

In the MBE and CVD processes, besides other variables, the thin filmgrowth rate is also affected by the concentrations of the startingmaterial inflows. To achieve a uniform surface smoothness of the thinfilms manufactured using these methods, the concentrations andreactivities of the starting materials must be kept equal on one side ofthe substrate. If the different starting materials are allowed to mixwith each other prior to reaching the substrate surface as is the casein the CVD method, the possibility of mutual reactions between thereagents is always imminent. Herein arises a risk of microparticleformation already in the in feed lines of the gaseous reactants. Suchmicroparticles generally have a deteriorating effect on the quality ofthe deposited thin film. However, the occurrence of premature reactionsin MBE and CVD reactors can be avoided, e.g., by heating the reactantsnot earlier than only at the substrates. In addition to heating, thedesired reaction can be initiated with the help of, e.g., plasma orother similar activating means.

In MBE and CVD processes, the growth rate of thin films is primarilyadjusted by controlling the inflow rates of starting materials impingingon the substrate. By contrast, the thin film growth rate in the ALDprocess is controlled by the substrate surface properties, rather thanby the concentrations or other qualities of the starting materialinflows. In the ALD process, the only prerequisite is that the startingmaterial is provided in a sufficient concentration for film growth onthe substrate.

The ALD method is described, e.g., in FI Patents Nos. 52,359 and 57,975as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Also in FI PatentsNos. 97,730, 97,731 and 100,409 are disclosed some apparatusconstructions suited for implementing the method. Equipment for thinfilm deposition are further described in publications Material ScienceReport 4(7), 1989, p. 261, and Tyhjiötekniikka (title in English: VacuumTechniques), ISBN 951-794-422-5, pp. 253-261.

In the ALD method, atoms or molecules sweep over the substrates thuscontinuously impinging on their surface so that a fully saturatedmolecular layer is formed thereon.

According to the conventional techniques known from FI PatentSpecification No. 57,975, the saturation step is followed by aprotective gas pulse forming a diffusion barrier that sweeps away theexcess starting material and the gaseous reaction products from thesubstrate. Intermixing of the successive reactant pulses must beavoided. The successive pulses of different starting materials and theprotective gas pulses forming diffusion barriers that separate thesuccessive starting materials pulses from each other accomplish thegrowth of the thin film at a rate controlled by the surface chemistryproperties of the different materials.

As known in the state of the art, dosing of precursors with a high vaporpressure, e.g. TMA and H₂O, makes it possible to use valves, which areoperated at ambient temperature. As explained in our earlier patents the“inert gas valving”, comprising a diffusion barrier and a drain operatedat ambient conditions, has made it possible to use the ALD process withthese high vapor pressure materials. In the following, inert gas valvingwill be also referred to in the abbreviated form “IGV”. It is disclosedand discussed in more detail in our copending U.S. patent applicationSer. No. 09/835,931 filed on Apr. 16, 2001, the content of which isherewith incorporated by reference. Today there is a growing interestfor the use of low vapor pressure solid precursors. The sourcetemperatures can rise up to above 500° C. This is the case for, e.g.,MnCl doping of ZnS phosphors. This puts stringent demands on the valvesemployed for controlling the dosing. Also the use of the IGV is somewhatcomplicated due to solid condensation of the precursor in the ambientoperated drain capillary, which can become blocked and by that endingthe proper function of the IGV, causing failure of the process. Insingle wafer cluster systems, down time for cleaning of draincapillaries and replacing source tubes after each run is not acceptable,contrary to the situation for batch type multi layer processing of thinfilm displays.

Due to the cyclic nature of ALD processing conventional valves cannot besolo adapted for this kind of a process. The aggressive source mediadestroy the valve rapidly at such conditions.

In the low vapor pressure dosing system for ALD systems, the cyclicinjection of a precursor into an ALD process requires a valvecontrolling the dosing. A mean time between failure of 20-40 millioncycles would be preferable for such a valve for production reasons. Inthe earliest ALD reactor constructions, solenoid-type valves were mainlyused employing valve seal materials of different kinds of elastomers orpolymers. Later on, pneumatic valves with metal membranes and metalseats have been used. When aggressive precursors are used at elevatedtemperatures, involving continuous closing actions, the result is rapidwear of the valve seal. Even pneumatically activated metal membranevalves release metallic particles into the process flow at suchconditions. A solenoid valve often creates abrasive products as a resultof the steam grinding the solenoid housing. For purity and safetyreasons small, convenient sized solenoids are not preferred in chemicalvapor deposition equipment.

Mass Flow Controllers (in the following abbreviated “MFC”) are widelyused for controlling the precursor dosing into conventional (notpulsing) CVD systems but they cannot be used for fast pulsing ALDsystems due to their slow response (long response times). At ambientconditions, pneumatic valves can tolerate only 0.2-4 million cycles dueto wear out of actuator seals and deformation of the valve steam andvalve seat. This is because of forces acting on the valve. At elevatedtemperatures the tolerance is close to zero. This is particularlydisadvantageous for ALD processing where the valve operates 100 to10,000 times during one process. The heat produced by the friction ofthe actuator piston movement increases quickly because of rapid pulsing.By contrast, in conventional chemical vapor deposition the precursorvalve operates only twice during one process.

Thus, as explained in earlier ALD patents and will have become apparentto a person skilled in the art from the above description, a fast actingdosing (pulsing) system with non-wear valves (low-particle level)characteristics would be essential for improved ALD processing.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate the problems ofthe prior art and to provide a novel method of growing a thin film ontoa substrate placed in a reaction chamber according to the ALD process.In particular, it is an object of the invention to provide a method inwhich the ALD process can be operated with increased reliability andreduced downtime caused by wear of the process equipment.

These and other objectives, together with the advantages thereof overknown processes, which shall become apparent from the followingspecification, are accomplished by the invention as hereinafterdescribed and claimed.

Generally, the present invention is based on the idea that leak-tightmechanical valves conventionally used for regulating the pulsing of thereactants, i.e. the flow of reactants from precursor sources to thereaction chamber, are replaced by an improved precursor dosing systemincluding. In particular, the flow of the reactant from the reactantsource to the reaction chamber is regulated with a regulating means,which provides choking of the reactant flow, while still allowing aminimum flow of said reactant between the vapour-phase pulses. At thesame time, the process is controlled by inert gas valving. Such a systemcan be implemented by arranging feeding inactive (mostly inert) gas intothe conduit which interconnects the reactant source with the reactionchamber (in the following called the “first conduit”) via a secondconduit, which is connected to the first conduit at a connection point.This inactive gas is fed during the time interval between thevapour-phase pulses of the reactant so as to form a gas phase barrieragainst the flow of vaporised reactants from the reactant source via thefirst conduit into the reaction chamber. The inactive gas from the firstconduit is withdrawn via a third conduit connected to the first conduit.The third conduit being maintained at a temperature equal to or higherthan the condensation of the vapour-phase reactant and being connectedto the first conduit at a point upstream of the second conduit. As aresult there is formed in the first conduit, along at least of length ofthat conduit, a gas flow which is directed in the opposite direction tothe reactant pulse feed. That gas flow will form a gas blow barrier.

As a result of the combination of inert gas valving with a valve systemwhich provides choking of the gas flow, while allowing a minimum,usually less than 5%, in particular less than 1% of the full flow (whenthe valve is fully open), the ALD process can be operated for sourcematerials which have low vapour pressure and which therefore need highvaporization temperatures.

More specifically, the method according to the present invention ischaracterized by what is stated in the characterizing part of claim 1.

Further details of the invention are apparent from the depending claims.

Considerable improvements are obtained by means of the invention. Thus,by means of the presented process, moving mechanical parts can beavoided in the area operated at temperatures above the precursorevaporation temperature. Reactant flow can be blocked without involvingany large forces in the operation of the pulsing valves. This willresult in negligible wear of valve steams and valve seats. The processis reliable and provides high productivity performance at low costs forany ALD system produced.

The operation of the present regulating mechanism, in the following alsocalled “inert gas valving” is reliable and it is not sensitive tovariations in the chemical character of the precursors. Since itincludes no mechanical moving parts, the investment costs and the needfor maintenance work is strongly reduced. As will be discussed in moredetail below, by the inert gas valving system, pulsing of reactants canbe carried out by using only one valve which controls the flow ofcarrier gas from the source of inactive or inert gas to the precursorsource. This valve can be kept at ambient temperature and it is not indirect contact with the reactants. By maintaining the temperature of thedraining conduit above the evaporation temperature of the reactant,condensation of the reactant in the hot zone of the apparatus can beavoided. There is no build-up of condensated reactants in the thirdconduit during the purge phase. All parts of the equipment are keptcleaner and less particles are formed which could be forwarded to thereaction chamber.

Next, the invention will be examined more closely with the aid of thefollowing detailed description and with reference to the appendeddrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a depicts a chart for the flow or reagents A and B and FIG. 1 bshows the principal configuration of a process lay-out according to thepresent invention.

FIG. 2 a is a sectional side view of a source with pre-tensioned metalmembrane and pneumatic control in an open position in accordance withthe present invention.

FIG. 2 b is a sectional side view of a source with pre-tensioned metalmembrane and pneumatic control in a closed position in accordance withthe present invention.

FIG. 3 is a sectional side view of a solenoid operated throttle valve inaccordance with the present invention.

FIG. 4 is a sectional side view of a disc valve source in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of growing a thin film onto asubstrate, in which method a substrate is placed in a reaction chamberand said substrate is subjected to surface reactions of a plurality ofvapor-phase reactants according to the ALD method to form a thin film.

The method comprises generally the steps of vaporising a reactant from areactant source, conducting the vaporised reactant to the reactionchamber via a first conduit, feeding said reactant into said reactionchamber in the form of vapour-phase pulses repeatedly and alternatelywith vapour-phase pulses of at least one other reactant, and causingsaid vapour-phase reactants to react with the surface of the substrateat a reaction temperature to form a thin film compound on saidsubstrate. The vaporised reactant can be conducted through a purifierbefore feeding it into the reaction chamber.

The present invention comprises basically replacing leak-tightmechanical valves conventionally used for regulating the pulsing of thereactants by an improved precursor dosing system including: ModulatedCarrier Gas flow, Precursor Transfer gas, Flow Ratio Sequencer device,Inert Gas valve with Hot Drain system.

In practice the IGV controlling part of the invention can be implementedby feeding inactive gas into the conduit interconnecting the reactantsource and the reaction chamber—that conduit will be called in thefollowing the “first conduit”—, via a second conduit, connected to thefirst conduit at a connection point. The inactive gas is then withdrawnfrom the first conduit via a draining conduit (in the following the“third” conduit) connected to the first conduit. The third conduitby-passes the reactor and it is maintained at a temperature equal to orhigher than the condensation of the vapour-phase reactant. By connectingthe third conduit to the first conduit at a point upstream of theconnection point of the second conduit it becomes possible to form a gasphase barrier which is opposite directed to the flow of vaporisedreactants from the reactant source via the first conduit into thereaction chamber.

Together with the IGV, the present invention employs a non leak proofvalve which will choke the flow of gas, although there will be allowed aminimum flow. Typically the flow is regulated with a valve having anopen and closed position such that the flow through the valve in theclosed position is {fraction (1/10)}-10,000 of the flow of the openposition of the valve. Preferably the flow is less than about 5%, inparticular less than 1% of the flow through the open valve. The valveused has, according to a preferred embodiment a response time of lessthan 100 ms, in particular less than 50 ms.

Among the valves, which can be used, the following can be mentioned:

-   a valve disc operated by the precursor transfer gas and gravity;-   a modified MFC solenoid operated throttle valve; and-   a pneumatic membrane type valve.

Another preferred embodiment comprises choking the same side carrier gasin parallel with the precursor pulse being injected at a time. It isalso possible to choke the opposite side carrier gas in parallel withthe precursor pulse being injected at a time.

The following description of the operation sequence, which refers toFIG. 1, starts from the system being in stand-by purge mode with carriergas MFC at 100% flow. The FRS is choked, the source temperature isactivated and the system is under vacuum. Thus, in accordance with FIG.1 the MFC 1 is feeding the carrier gas via a conduit 2 into the reactionspace 3, providing for the purge of the reaction space when theprecursor injection is off. The reaction space is emptied by the vacuumpump 10.

The Flow Ratio Sequencer FRS 4 is choking the precursor transfer inertgas flow during the purge cycle, into a sufficient flow of 0.05 sccm toa few sccm that will protect the upstream gas line 5 and the FRS fromproblems related to condensation of the precursor into the upstream gaslines. The FRS controlled inert gas leak will direct the precursordiffusing from the source 6 via the Hot drain capillary 8 into thevacuum pump 10 assisted by the Diffusion Barrier 7 gas flow, preventingthat said precursor of entering the reaction space 3 at a time when theother precursor is interacting in the reaction space.

Simultaneously with the carrier gas MFC being choked the precursortransfer flow becomes non-choked by the FRS and the inert gas will flushthe source, overcome the diffusion barrier flow, transferring theprecursor to the reaction space. The reduced flow of the carrier gasduring the precursor pulse increases the concentration of the precursordoze thus aiding the saturation of the surface with the precursormolecules. In order to improve the spreading of the precursoradditionally into the reaction space the flow of both side carrier gasMFC's can be reduced during the precursor pulse. Example of a flowdiagram illustrating this principle is presented in FIG. 1 b.

The diffusion barrier is formed by inert or inactive gas in the conduitinterconnecting the reactant source with the reaction chamber. These gasbarriers are generated in the time interval between two successivepulses of the same reactant gas. The time interval typically includes apurge pulse, a pulse of another reactant and a further purge pulse.

The Flow Ratio Sequencer FRS 4 contains a modulating actuator,preferably a fast acting piezoelectric valve or MFC without any abrasivemoving parts that could produce particles to enter the gas flow. Thecontroller should be able to control the gas flow with response timesbelow 100 ms.

When the FRS is choked a small controlled leak of an inert gas is fedinto the heated Precursor Source 6 via conduit 5 preventing thediffusion of the precursor material into the upstream conduit and theFRS where condensation of the material would occur. The leaking transfergas with a small amount of the precursor material is conducted intoconduit 9 containing the Hot Drain capillary 8 which controls the InertGas Valve flow in the Diffusion Barrier path 7 preventing the precursorof entering the reaction space in a non-controlled way.

As mentioned above, the non-fully closing valve used in the inventionwill strongly choke the gas flow although it will still allow a smallthrough-flow (a gas leak flow) in “closed” position. The reduction ofthe gas flow from open to closed position must nevertheless be such thatit is possible essentially to prevent intermixing of reactor pulses byusing the IGV. As mentioned above, the flow through the valve in theclosed position is preferably about {fraction (1/10)}-10000 of the flowof the open position of the valve.

In accordance with the invention there are many alternative solutionsfor placing the non-closing valve in the process. For example thenon-fully closing valve can be placed inside the flow ratio sequencer 4or between the flow ratio sequencer 4 and the precursor source 6.

Further, the non-fully closing valve may be placed in the normal flowdirection after the precursor source 6 or even in the conduit 9 for hotdrain capillary 8.

One kind of FRS device is available from Engineering MeasurementsCompany, Longmont USA, with brand name of Mach-One. Pat. WO 98/37343.

It can be used as a ratio controlled valve with a leak rate of 0.03 secmproviding a turndown ratio of for example 100:1 (open:close), enablingby that a non-closing type valve when operated combined with the inertgas switching valve.

When the FRS is used as the carrier gas mass flow controller, operatedin synchrony with the opposite pulsing valve, it enables additionally agradient free non-diluted distribution of the precursors into thereaction space.

Other suppliers of suitable fast acting valves suitable for Flow Ratiocontrolled ALD valve system applications are: Horiba Ltd, Kyoto, Japan.Fujikin Incorporated, Japan, Brooks Instruments, Hatfield, USA.

The basic demand for the component is non-fully closing, fast responding(<100 ms), with for example 100:1 turndown ratio, preferably>20 millioncycle durability proven. There are, of course, many other suppliers ofsuch valves that could be modified for this application withoutdeparting from the scope of this invention. The scope of this inventionis the combination of a flow ratio controlling fast acting valve,combined with the Inert Gas Valve providing together a novel, productiveALD process for growing a thin film onto a substrate.

Another embodiment comprises pneumatic “all metal” membrane type valves,(FIGS. 2 a and 2 b) which enable the use of the valve in stronglyelevated temperatures. Combined with the IGV no leak tight performanceis needed and by that no large closing force.

Other actuator elements than solenoid steams can be used. The embodimentin FIG. 3 shows a modified solenoid operated throttle valve, based on acentrally fixed spring mounted solenoid steam and is modified from anordinary MFC provided by Brooks Instruments.

The embodiment of FIG. 4 shows a disc valve where the closing disc ismoved by the: dosing precursor transfer inert gas pulse, transferringthe precursor into the reaction space. When the transfer gas pulse isclosed, the disc will set on the valve seat by gravity, thus closing thesource opening with a sufficient ratio (>100-1) providing closingconditions for the IGV.

In the drawing, the following reference numerals are used:

-   31. Valve disc-   32. Precursor-   33. Vaporized precursor-   34. Heating envelope-   35. Flow Ratio Sequencer-   36. Transfer gas injection conduit-   37. Transfer gas inlet-   38. Valve body-   39. Conduit to inert gas valve

According to the invention it is also essential that the regulatingmeans, which provides for choking of the reactant is operated inconjunction with inert gas valving (IGV). The IGV is operated asfollows:

Inactive gas is used for forming a gas phase barrier, which preventsleaking of reactant from the reactant feed conduit into the reactionchamber during purging and during feed of another reactant. Thus, thepresent invention comprises generating a gas phase barrier in theconduit interconnecting the reactant source and the reaction chamber atsome point of the conduit either before or after the purifier. The gasphase barrier preferably comprises a flow of inert (in the followingmore generally “inactive” gas) which is directed in the oppositedirection to the flow of vaporized reactant. The point at which theinactive gas is introduced from a second conduit into the first conduitis positioned downstream (with respect to the normal flow direction ofthe reactant gas from the source to the reaction chamber) from the pointat which the inactive gas is withdrawn from the conduit. Thus, at leastfor some length of the first conduit, the inactive gas fed via thesecond conduit is conducted in opposite direction to the reactant flow.

Summarizing, the barrier zone of the first conduit (which comprises thelength of the first conduit between the connecting points of the secondand the third conduits) exhibits a gas flow, which is generally directedtoward the reactor during pulsing and toward the reactant source duringthe purge cycle. There may also be formed a barrier zone in the thirdconduit for reducing the waste of reactant during pulsing.

In the present context, the term “inactive” gas is used to refer to agas which is admitted into the reaction space and is capable ofpreventing undesired reactions related to the reactants and thesubstrate, respectively. In the method according to the invention, theinactive gas is also used advantageously as the carrier gas of thevapor-phase pulses of the reactants and, in particular, for providing agas barrier to the flow of reactant residues into the reaction chamberduring the purging of the reaction chamber. Of inactive gases suited foruse in the method, reference can be made to inert gases such as nitrogengas and noble gases, e.g., argon.

The “first conduit” is a pipe made from, e.g., metal or glass whichinterconnects the reactant source with the reaction chamber. As willexplained below, the first conduit is provided with at least twoconnecting pipe branches, one for introducing inactive gas (connected tothe conduit at an inactive gas feed nozzle) and another for withdrawinginactive gas.

According to a preferred embodiment, the third conduit comprises an opengas flow channel. The term “open” means that the gas flow channel is notprovided with a valve which can be completely closed. It can, however,be provided with flow restrictions such as capillars, which reduce thecross-section of the conduit. The third conduit, which by-passes thereaction chamber, drains the first conduit. In order to avoidcondensation, it is maintained at a temperature equal to or higher thanthe condensation of the vapour-phase reactant. Preferably, thetemperature is equal to or lower than the reaction temperature.

In some embodiments, in particular when there is a solid (powdery)reactant source, it is preferred to have a filter between the reactantsource and the reaction chamber. In such embodiments, the second conduitcan be connected to the first conduit at a point between the filter andthe reaction chamber so as to create a one-way gas flow over the filter.In this embodiment, the gas phase barrier is formed between a purifyingmeans and the reaction chamber. The second conduit can also be connectedto the first conduit at a point between the reactant source and thefilter.

The third (draining) conduit can be connected to the first conduit at apoint between the connection point between the first conduit and thesecond conduit and the reactant source.

The unreacted vapour-phase reactants are withdrawn from the reactionchamber via an outlet conduit, and the third conduit is connected tothat outlet conduit. It is, however, also possible to have the thirdconduit connected to a separate evacuation means.

According to a preferred embodiment, essentially all of any vapour-phasereactant from the reactant source is conducted via the third conduit tothe drain between the feed of vapour-phase reactant pulses into thereaction chamber. Since the third conduit is not closed by a valveduring the pulsing of reactants from the reactant source, there is asmall flow of precursor from the source to the third conduit duringpulsing of the reactant.

The flow of inactive gas through the third conduit is generally smallerthan the flow of gas through the first conduit. However, on someoccasion there may arise a need for growing the flow through thedraining conduit. Typically the flow through the third conduit is aboutone fifth of that in the first conduit. Preferably it is less than 15%,in particular preferably 10% or less of the flow via the first conduitinto the reaction chamber. In order to minimize reactant losses via thethird conduit during pulsing, a fourth conduit can be connected to thethird conduit. The fourth conduit is used for feeding inactive gas intothe third conduit in the opposite direction to the flow of the gaswithdrawn from the first conduit. Thus, the inactive gas fed from thefourth conduit will “push” the reactant vapours back towards the firstconduit. A further advantage of separate inactive gas introduction intothe third conduit is that the inert gas will be diluted with respect tothe precursor, which will reduce the tendency of condensation.

In order to adjust the relative flow rates of the first and the thirdconduits, it is preferred to incorporate flow restrictors into the thirdconduit. Such a flow restrictor can be a static restriction such as acapillary portion which can be exchanged depending on the conditions.Since the static restriction contains no moving parts, the durability ofit is good.

By feeding the inactive gas from the fourth conduit to a point above therestriction point (i.e. between the flow restriction and the connectionpoint between the first conduit and the third conduit) it becomespossible to form a separated gas barrier zone during pulsing which willreduce the loss of precursor.

1. A method of growing a thin film onto a substrate placed in a reactionchamber according to the ALD method, said method comprising the stepsof: vaporizing a reactant from a reactant source maintained at avaporizing temperature; conducting the vaporized reactant to thereaction chamber via a first conduit; regulating the flow of saidreactant so as to feed the reactant into said reaction chamber in theform of vapor-phase pulses repeatedly and alternately with vapor-phasepulses of at least one other reactant; causing said vapor-phase reactantto react with the surface of the substrate at a reaction temperature toform a thin film compound on said substrate; feeding inactive gas intosaid first conduit via a second conduit, connected to the first conduitat a connection point, during the time interval between the vapor-phasepulses of the reactant so as to form a gas phase barrier against theflow of the vaporized reactant from the reactant source via the firstconduit into the reaction chamber; and withdrawing the inactive gas fromsaid first conduit via a third conduit connected to the first conduit,said third conduit being maintained at a temperature equal to or higherthan the condensation of the vapor-phase reactant and being connected tothe first conduit at a point upstream of the second conduit; and whereinregulating the flow of said reactant comprising choking of the reactantflow while still allowing a minimum flow of said reactant between thevapor-phase pulses.
 2. The method according to claim 1, whereinregulating the flow of said reactant comprises operating a valve havingan open and closed position such that the flow through the valve in theclosed position is {fraction (1/10)}-10,000 of the flow of the openposition of the valve.
 3. The method according to claim 1, whereinregulating the flow of said reactant comprises operating a valve with aresponse time less than 100 ms.
 4. The method according to claim 1,wherein regulating the flow comprises operating a valve disc operated bya precursor transfer gas and gravity.
 5. The method according to claim1, wherein regulating the flow comprises operating a modified MECsolenoid operated throttle valve.
 6. The method according to claim 1,wherein regulating the flow comprises operating a pneumatic membranetype valve.
 7. The method according to claim 1, wherein the same sidecarrier gas is choked in parallel with the precursor pulse beinginjected at a time.
 8. The method according to claim 1, comprisingchoking an opposite side carrier gas in parallel with the vapor-phasepulse.
 9. The method according to claim 1, wherein regulating the flowof said reactant comprises operating a non-fully closing valve that isplaced inside a flow ratio sequencer.
 10. The method according to claim1, wherein regulating the flow of said reactant comprises operating anon-fully closing valve that is placed between a flow ratio sequencerand a precursor source.
 11. The method according to claim 1, whereinregulating the flow of said reactant comprises operating a non-fullyclosing valve that is placed after a precursor source in the normal flowdirection.
 12. The method according to claim 1, wherein regulating theflow of said reactant comprises operating a non-fully closing valve thatis placed in a conduit connected to the first conduit to provide a hotdrain capillary.
 13. The method according to claim 1, wherein feedinginactive gas into the first conduit comprises feeding, at least for somelength of the first conduit, the inactive gas in opposite direction tothe reactant flow.
 14. The method according to claim 1, comprisingmaintaining the second conduit at a temperature equal to or lower thanthe reaction temperature.
 15. The method according to claim 1, whereinthe third conduit comprises an open gas flow channel.
 16. The methodaccording to claim 1, wherein feeding the inactive gas into the firstconduit comprises feeding the inactive gas into the first conduit at apoint downstream from the connection point at which the third conduit isconnected to the first conduit to provide a flow of inactive gas whichis directed in the opposite direction to the reactant flow in the firstconduit.
 17. The method according claim 1, conducting the vaporizedreactant through a purifier that is provided in the first conduit and isselected from the group consisting of a filter, a ceramic molecularsieve and an electrostatic filter capable of separating dispersed liquidor solid droplets or particles or molecules of a minimum molecular sizefrom the reactant gas flow.
 18. The method according to claim 1,comprising conducting the vaporized reactant through a is an activepurifier comprising functional groups capable of reacting withcomponents present in the reactant gas flow.
 19. The method accordingclaim 1, wherein vaporizing a reactant from a reactant source comprisesfreeing a solid or liquid reactant from suspended liquid or solidparticles in a filter placed in the first conduit along the flow path ofthe reactant to the reactant chamber.
 20. The method according to claim1, comprising conducting the vaporized reactant through a filter that islocated downstream of where the second conduit is connected to the firstconduit.
 21. The method according to claim 1, comprising conducting thevaporized reactant such that there is a one-way gas flow over a filter.22. The method according to claim 1, wherein feeding inactive gas intothe first conduit via the second conduit comprises forming the gas phasebarrier between a filter and the reaction chamber.
 23. The methodaccording to claim 1, wherein feeding inactive gas into the firstconduit via the second conduit comprises feeding the inactive gas to thefirst conduit at a point between the reactant source and a filter. 24.The method according to claim 1, wherein withdrawing the inactive gasfrom said first conduit comprises withdrawing the inactive gas at apoint between the connection point between the first conduit and thesecond conduit and the reactant source.
 25. The method according toclaim 1, wherein the reactant source is freely communicating with thefirst conduit.
 26. The method according to claim 1, comprisingwithdrawing the unreacted vapor-phase reactants from the reactionchamber via an outlet conduit that is connected to the third conduit.27. The method according to claim 1, comprising evacuating the thirdconduit through a separate evacuation device connected to the thirdconduit.
 28. The method according to claim 1, wherein conducting thevaporized reactant to the reaction chamber comprises using the inactivegas as a carrier gas for the vaporized solid or liquid reactant.
 29. Themethod according to claim 1, comprising withdrawing essentially all ofany vapor-phase reactant from the reactant source that is conducted viathe third conduit to a drain between the feed of vapor-phase reactantpulses into the reaction chamber.
 30. The method according to claim 1,comprising maintaining a condensation vessel that is connected to thethird conduit at a lower pressure and/or temperature in order to providecondensation of vaporized reactant residues.
 31. The method according toclaim 1, comprising feeding inactive gas into the third conduit througha fourth conduit.
 32. The method according claim 31, comprising reducingthe amount of gas withdrawn from the first conduit by feeding inactivegas into the third conduit.
 33. The method according to claim 31,comprising feeding the inactive gas during pulsing of the reactant. 34.The method according to claim 31, wherein the inactive gas is fed intothe third conduit at a point above any flow restrictor.
 35. The methodaccording to claim 1, wherein inactive gas is fed into the reactionchamber between the vapor-phase pulses of said reactants.
 36. A methodof using a non-fully closing valve in a method for growing a thin filmonto a substrate placed in a reaction chamber according to the ALDmethod, said method comprising the steps of: vaporizing a reactant froma reactant source maintained at a vaporizing temperature; conducting thevaporized reactant to the reaction chamber via a first conduit;regulating the flow of said reactant so as to feed the reactant intosaid reaction chamber in the form of vapor-phase pulses repeatedly andalternately with vapor-phase pulses of at least one other reactant;causing said vapor-phase reactants to react with the surface of thesubstrate at areaction temperature to form a thin film compound on saidsubstrate; feeding inactive gas into said first conduit via a secondconduit, connected to the first conduit at a connection point, duringthe time interval between the vapor-phase pulses of the reactant so asto form a gas phase barrier against the flow of vaporized reactant fromthe reactant source via the first conduit into the reaction chamber; andwithdrawing the inactive gas from said first conduit via a third conduitconnected to the first conduit, said third conduit being maintained at atemperature equal to or higher than the condensation of the vapor-phasereactant and being connected to the first conduit at a point upstream ofthe second conduit; and wherein regulating the flow of said reactantcomprising choking of the reactant flow while still allowing a minimumflow of said reactant between the vapor-phase pulses.